Conversion of natural gas to liquid hydrocarbons (“Gas To Liquids” or “GTL” process) is based on a 3 step procedure consisting of: 1) synthesis gas production; 2) synthesis gas conversion by FT synthesis; and 3) upgrading of FT products (wax and naphtha/distillates) to final products.
The Fischer-Tropsch reaction for conversion of synthesis gas, a mixture of CO and hydrogen, possibly also containing essentially inert components like CO2, nitrogen and methane, is commercially operated over catalysts containing the active metals Fe or Co. Iron catalysts are best suited for synthesis gas with low H2/CO ratios (<1.2), e.g. from coal or other heavy hydrocarbon feedstock, where this ratio is considerably lower than the consumption ratio of the FT reaction (2.0-2.1). A variety of products can be made by the FT-reaction, but from supported cobalt, the primary product is long-chain hydrocarbons that can be further upgraded to products like diesel fuel and petrochemical naphtha. By-products can include olefins and oxygenates.
To achieve sufficient catalytic activity, it is customary to disperse the Co on a catalyst carrier, often referred to as the support material. In this way, a larger portion of Co is exposed as surface atoms where the reaction can take place. Supported cobalt catalysts are the preferred catalysts for the FT synthesis. The most important properties of a cobalt FT catalyst are the activity, the selectivity usually to C5 and heavier products, i.e. C5+, and the resistance towards deactivation. The physical strength and chemical robustness of the catalyst and support are also crucial. Normally, the catalyst is deployed in a slurry type, fluidized bed or fixed-bed reactor when used, but other reactor types like a microstructured reactor have been proposed. In a slurry reactor the average catalyst particle size can be between 20 and 200 μm.
All industrially operated catalysts, possibly with very rare exemptions, experience deactivation, i.e. a decline of the catalyst activity with time-on-stream (TOS). Often, the catalyst must be exchanged for a fresh one after some time of operation, typically between 0.5 and 5 years. For some reactions experiencing very rapid deactivation, a form of continuous or semi-continuous regeneration is needed. This is typical for FCC (fluid catalytic cracking) in the refinery where coke must be burned off after seconds of operation. However, continuous deactivation is also seen as a result of pick-up of impurities from the oil. In catalytic reforming to give gasoline the active Pt/Re or Pt/Sn system must be regularly regenerated by re-dispersing the active platinum on the support. For a cobalt Fischer-Tropsch catalyst, a study of a deactivated catalyst that has been operated in a slurry bubble column is reported in Applied Catalysis A: General, volume 354, pages 102-110, 2009. The main conclusion is that long-term deactivation is caused by carbon rich deposits.
Moderate experience regarding catalyst regeneration has been gained from operation of full-scale commercial (2000-20000 bpd production) or semi-commercial size Fischer-Tropsch reactors (200-2000 bpd) using cobalt type catalysts, particularly when using a slurry reactor type operation. The catalyst contains expensive cobalt and frequently also exotic promoters like platinum or rhenium. These are extremely expensive and due to the large size of modern GTL plants can also constitute a major portion of the world production. After unloading a deactivated catalyst it therefore becomes mandatory to reclaim as much as possible of the metals. These can be used in further catalyst production. Metals reclamation is usually a complicated process that involves multiple steps like chemical extraction or complexation. Further, the catalyst in itself will of course be destroyed. Therefore, a much more attractive solution would be to regenerate the catalyst for further use.
There are two main approaches to regeneration, in situ and ex situ, meaning inside the FT-reactor itself or separate from the reactor. In situ implies stopping the FT-reaction and using special conditions as to gas composition, and possibly temperature and pressure. Special regeneration configurations that we denote in situ in the present context are using part of the reactor volume in a continuous regeneration, or doing the same by taking a side stream from a slurry or fluidized-bed reactor that is exposed to regeneration conditions and continuously deployed into the reactor again. Although in situ regeneration has certain merits, it is seriously hampered by lack of flexibility in the conditions that can be applied. For instance, in a slurry FT-reactor one cannot use elevated temperature and/or oxygen in order not to destroy the liquid phase with possible severe consequences on the slurry operation. Further, design and operation of the reactor will be very complex to an extent that generally makes in situ regeneration impractical.
The use of hydrogen or a hydrogen-rich gas is an option that has been proposed, see e.g. EP0319625 where in situ regeneration of a cobalt FT-catalyst in low-temperature flowing hydrogen has been disclosed. However, the efficiency of such a regeneration is questionable, as deposited heavy hydrocarbons will not be removed to the extent needed.
In WO 2008/139407, a method for ex situ regeneration is described. The spent cobalt FT catalyst is first subjected to a dewaxing treatment, an oxidation treatment at a pressure of 4 to 30 bar(a) followed by a reduction treatment. The dewaxing is described as hydrogenolysis, solvent wash or extraction, or combinations thereof. Unfortunately, in WO 2008/139407, the effect of regeneration per se is not shown, only the relative results of using varying pressures during the oxidation stage.